IBM's International Components for Unicode1 version 3.2 Layout Engine (released Nov. 2004) includes OpenType layout support for the scripts with stars shown above, with the exception of Thai. Thai is supported, but Thai OpenType tables are not yet supported. As is the case with Pango, the Arabic support includes full OpenType support for Nashtaliq.
Eric Mader, the author of ICU's Layout Engine, told me that some bugs probably still remain in the Indic scripts. The Khanda Ta2 problem is a known problem in Bengali Unicode support which requires resolution from the Unicode Consortium.
As mentioned, Eric Mader was responsible for porting Indic OpenType support to Pango a few years ago. Since that time, Owen Taylor of RedHat and Mader have collaborated informally in synchonizing bug fixes between Pango and ICU. As a result, users should expect to see very similar results from both Pango and ICU where these engines have overlapping script support.
ICU just got Khmer support at the end of December, 2004. Support for Lao, Myanmar, Tibetan, and Sinhala is still missing. Mader expects that porting Pango's Sinhala support to ICU will probably not be much trouble.
Note that on Linux, the OpenOffice suite uses ICU's Layout Engine, although what was used at the time of this writing was an older 2.x version, not the latest 3.2 version. On Windows, OpenOffice uses Uniscribe.
1. International Components for Unicode, http://oss.software.ibm.com/icu/.
2. Peter Constable, Encoding of Bengali Khanda Ta in Unicode, Public Review Issue #30, http://www.unicode.org/review/pr-30.pdf.